To BW's credit I have received a speedy response to my email to them. Reproduced below for those interested/affected by the stoppages.
"We apologise for the confusion regarding the stoppages you listed below.
Lock 16W to 17W:
The retaining wall which collapsed is currently in the process of being rebuilt to re-open the towpath, the rubble will be removed from the canal and towpath during these works. The canal level should not have been low today and we thank you for bringing this to our attention, we are currently investigating why it was low and hope to have it refilled tomorrow.
Halls Lock 20W:
Unfortunately due to the works being carried on Halls Bridge by contractors for British Gas, the planned works on the lock have been delayed but are due to start on Monday 19 March, this will not affect the completion date of March 30th.
Halls Bridge, Lock 20W:
Works are currently being carried out by British Gas contractors. The navigation is still open and the contractors are escorting boats through whilst this is on-going.
There shouldn’t be any problems with your passage through this navigation this weekend, however, if you would like further clarification or have any other enquiries please feel free to contact us on 01782 785703"